FRIDAY, May 1 (HealthDay News) -- The number of confirmed swine flu cases in the United States has now reached 141 in 19 states, federal health officials reported on Friday.

"That's up eight states since yesterday," Dr. Anne Schuchat, interim deputy director for the U.S. Centers for Disease Control and Prevention's science and public health program, said during a teleconference. "More and more communities are being affected, and more people are being directly impacted by the H1N1 novel virus we are seeing this year. Cases continue to occur."

Nancy Cox, chief of the CDC's influenza division, also had some welcome news on the nature of the virus itself. She said during the teleconference that a preliminary analysis of the H1N1 strain finds it lacks certain "virulent characteristics" that made the 1918 flu pandemic strain so deadly.

And the new Secretary of Health and Human Services, Kathleen Sebelius, has made the decision to buy 13 million more courses of antivirals to replenish the antiviral stockpile, Schuchat said. "We don't know if we are going to need them, we just wanted to be ready," she said.

In addition, in the last 24 hours, the United States has shipped 400,000 regimens of antivirals to Mexico, believed to be the source of the global outbreak, at the request of the Mexican government, Schuchat added.

Meanwhile, President Barack Obama on Friday urged Americans to stay calm, noting that it was not clear whether the global outbreak of the never-before-seen flu strain was any worse than "ordinary flus." But, he added, agencies across the U.S. government are preparing for the worst, according to the Associated Press.

If swine flu "is relatively mild on the front end, it could come back in a more virulent form during the actual flu season," he said at the end of a Cabinet meeting.

Elsewhere, the World Health Organization reported Friday that the number of confirmed cases worldwide has risen from 257 to 365 in 13 countries, with 10 deaths. And Asia announced its first case, in Hong Kong. Officials there quarantined an entire tourist hotel where the victim, a traveler from Mexico who entered via Shanghai, had stayed Thursday night before getting sick, according to The New York Times.

The news out of Mexico appeared brighter. Late Thursday, Health Secretary Jose Angel Cordova said the number of new swine flu cases seemed to be leveling off.

"The fact that we have a stabilization in the daily numbers, even a drop, makes us optimistic," Cordova said at a news conference. "Because what we'd expect is geometric or exponential growth. And that hasn't been the situation."

Still, Mexico braced for a shutdown of all non-essential services, including all schools, through Tuesday as authorities sought to limit further infections in that country, where the virus has stricken 156 people, killing nine.

In the United States, the CDC says there are 50 confirmed cases in New York, 28 in Texas, 13 in California, 16 in South Carolina, five in New Jersey, four each in Arizona and Delaware, three each in Illinois and Indiana, two each in Colorado, Kansas, Massachusetts, Michigan and Virginia, and a case each in Kentucky, Minnesota, Nebraska, Nevada and Ohio.

On Monday, a 23-month-old Mexican boy who had traveled to Houston for medical treatment died, becoming the first -- and so far, only -- fatality in the United States.

Speaking at a press conference on Thursday, CDC Acting Director Dr. Richard Besser said that while most cases appear to be mild, "six of the cases have been hospitalized, including the unfortunate case we reported yesterday of the child in Texas who passed away."

And scientists were racing to produce a vaccine against the new flu strain, but the shots -- if needed at all -- wouldn't be available until fall at the earliest, U.S. health officials said Thursday.

On Friday, U.S. health officials told reporters that six countries -- the United States, Canada, the Netherlands, Mexico, Germany and New Zealand -- have all shared samples of the virus for testing to further the vaccine effort.

"The good news is that the genes of all of the viruses we have examined to date are 99 to 100 percent identical," Cox said. "This means that it will be somewhat easier for us to produce an influenza vaccine."

"We are aggressively taking the very early steps that are necessary for vaccine manufacture should a decision be made go ahead and ramp up to full-scale production," Cox added.

The current plan is to have vaccine manufacturers complete production of next year's seasonal flu vaccine, then, if necessary, switch to the production of the H1N1 vaccine, Schuchat said.

The flu strain is a combination of pig, bird and human viruses, prompting worries from health officials that humans may have no natural immunity to the pathogen.

"Consumers who purchase products to treat the novel 2009 H1N1 virus that are not approved, cleared or authorized by the FDA for the treatment or prevention of influenza risk their health and the health of their families," Michael Chappell, acting FDA Associate Commissioner for Regulatory Affairs, said in a news release. "In conjunction with the Federal Trade Commission, the FDA has developed an aggressive strategy to identify, investigate and take regulatory or criminal action against individuals or businesses that wrongfully promote purported 2009 H1N1 influenza products in an attempt to take advantage of the current flu public health emergency."

As with the previously tested strains of the swine flu virus, new testing has found that the pathogen remains susceptible to the two common antiviral drugs Tamiflu and Relenza, according to the CDC.

In the United States, nearly 300 schools have been closed in response to the outbreak, according to news reports. Texas has postponed all public high school sports and academic competitions until at least May 11.

On Wednesday, the WHO raised the swine flu epidemic level from 4 to 5, signifying that a pandemic is imminent, and urged countries to implement their pandemic plans. And on Thursday, WHO said it would cease calling the virus "swine flu," using instead its scientific name -- H1N1 influenza A -- to help reduce confusion over the danger posed by pigs. Pork consumption does not transmit infection.

Along with Hong Kong, Mexico and the United States, swine flu infections have now been reported in Canada, Denmark, New Zealand, Britain, Germany, Spain, Israel, The Netherlands, Switzerland and Austria.

SOURCES: May 1, 2009, teleconference with Anne Schuchat, M.D., interim deputy director, science and public health program, U.S. Centers for Disease Control and Prevention, and
Nancy Cox, Ph.D., chief, influenza division, National Center for Immunization and Respiratory Diseases, U.S. Centers for Disease Control and Prevention; April 30, 2009, teleconference with Richard Besser, M.D., acting director, U.S. Centers for Disease Control and Prevention; Associated Press; New York Times